This invention relates to turbomachines, and more particularly to labyrinth seals for rotor and stator labyrinths.
Labyrinth seals used as a seal between rotating and static parts of axial turbomachines are known in general from the state of the art. Unexamined patents (Offenlegungsschriften) DE-A1-35 23 469, EP-A1-982 475, EP-A1-799 973, or EP-A1-943 784, the disclosures of which are incorporated herein by reference, describe various embodiments of such turbomachines with sealing strips, and labyrinths located in between the latter. These sealing strips usually are mortised into a peripheral groove of the rotor and stator, as shown, for example, in FIG. 1 of EP-A1-982 475. Depending on the pressure differential occurring across the sealing strips, the labyrinth seal can be constructed in different ways. There are, for example, simple and double seals. Especially in the case of large pressure differentials, several sealing strips distributed over the length of the labyrinth also can be used as seals.
Several factors limit the geometrical arrangement of the sealing strips on the rotor or stator. During non-stationary processes, i.e., for example, during start-up or shut-down or when changing loads, the thermal load on the sealing strips is very high because of the changing temperature fields and resulting temperature gradients. This creates thermal tensions, particularly at the surface, and in this way causes cyclical fatigue. The peripheral grooves hereby act as notches that increase the axial tension component. In order to reduce the notch effect of a groove, the entire labyrinth can be set off from the rest of the component, for example, by increasing the height of the labyrinth part and by appropriately designed transition radii; this reduces the load primarily on the first and last groove. For the remaining grooves, a certain relief effect is achieved by the respective adjacent grooves, i.e., the notch factor of a groove within such an arrangement of several grooves is lower than that of a single, isolated groove with the same geometry.
It is known from various studies that the optimal distance between two grooves with respect to mutual load relief within an arrangement of several consecutive grooves is generally smaller than the distance that should be selected for functional reasons between two sealing strips in a labyrinth seal. This means that in the case of high transient thermal mechanical loads, in particular towards the center of a labyrinth section, both cyclical life span problems as well as severe deformations of the individual fastening grooves occur as a result of the notch effect of the fastening grooves. Since the notch effect acts on the axial tension component, a strong axial deformation of the groove occurs during each operating cycle. The deformation may be such as to even create inelastic sections, which on the one hand causes a continuous gradual detachment of the mortised sealing strips, and on the other hand, also causes a decrease in the preload force achieved during the mortising. In the end, this deformation causes a loss of the corresponding sealing strip. Because of the cyclical fatigue, superficial fissures in the groove base of the sealing strip groove also must be expected.
For this reason, K. Schrxc3x6der suggests in Dampfkraftwerke (3rd Vol., Part B, Springer Verlag, 1968, p. 68-69) to cut relief grooves between two sealing strips. This has the objective of compensating tensions caused by the mortising and reducing the thermal load when a plate is brushed against. Such an arrangement of individual relief grooves that have more than twice the depth than sealing strip grooves has a limited positive influence on the thermal tension reduction. However, a specific reduction of the thermal tensions between the sealing strips is not possible or is possible only to a limited degree with this type of relief grooves, in particular, because an increase in the number of relief grooves requires that a specific wall thickness must be preserved in any case between two relief grooves. This means that these relief grooves in no way can be arranged in an optimal manner. As a rule, such designs result in a shift of the problems, not in a solution. In particular, deeper cuts should be avoided if only to prevent a swirling of the leakage current in these cuts and the associated heating of the flowing medium. In addition, individual relief grooves with the same depth as or deeper than the sealing strip grooves in general have a poorer fatigue-stress concentration factor than the sealing strips, so that the fatigue problem shifts to the relief groove. This is very undesirable, in particular, for seals on shafts.
It is an objective of this invention to avoid the described disadvantages. The invention has the objective of optimizing a known labyrinth seal in such a way that the thermal tensions or deformations between two sealing strips can be controlled in a targeted manner in order to avoid the above-mentioned damage mechanisms, and that an additional heating of the component by a swirling of the leakage current can be avoided.
According to the invention, these objectives are achieved in a labyrinth seal wherein the axial stiffness of the rotor or stator between the two sealing strips is substantially steady.
In a first embodiment, at least one flat relief groove is set between two sealing strips and extends over a larger area between the two adjacent sealing strips. It would also be conceivable to provide a plurality of flat relief grooves whose longitudinal extension is correspondingly smaller. In a preferred embodiment, the depth of the relief grooves is reduced to such an extent that only one corrugated surface is located between the two adjoining sealing strips. The desired objective can be realized advantageously in this manner, whereby the increased number of relief grooves enables a very targeted reduction in tension. Abrupt fluctuations in stiffness between the grooves and the rotor or stator are avoided or are kept as small as possible. This advantageously reduces the notch factor.
In another embodiment, it is also possible to use the previously known deep relief grooves if they are filled with a suitable elastic material. This measure is used for the same, above-mentioned purpose of controlled, even distribution of the axial tension over the rotor or stator section between two sealing strips. The relief grooves hereby can be filled entirely or partially with the filling material. In the simplest case, the same wire that is also used for mortising in the sealing strip also can be used as a filling material. In general, any materialxe2x80x94preferably in wire formxe2x80x94that has the required elasticity and long-term stability at the operating temperature of the seal can be used.
All embodiments are also advantageous because, in addition to an improved tension absorption or distribution, they also prevent a damaging vortex generation in the labyrinth or within the relief grooves. Such a vortex generation may result in an undesired heating of the flow medium and therefore of the entire rotor or stator section.